Field of the Invention
The invention relates in general to a wireless communication system, and more particularly, to a packet detection technology.
Description of the Related Art
A wireless local area network (WLAN) system transceives data in a unit of packets. To lower possibilities of collisions between packets and to enhance transmission quality, two successive packets are transmitted with a constant time interval in between. For example, 802.11n specifications define a short interframe space (SIFS) interval in length of 16 μs, and a reduced interframe space (RIFS) interval in length of 2 μs.
FIG. 1 shows an example of a timing relationship between two 802.11n packets and corresponding operations at a receiving end according to the prior art. In the example, the packet P1 is completely delivered to the receiving end at the time point t10, and the receiving end finishes decoding the packet P1 at the time point t11 and starts searching for a next packet. A preamble at the beginning of each packet, e.g., a shaded area at the beginning of a packet P2, serves as reference for the receiving end to determine the presence of a packet. In general, the receiving end has no way of learning in advance whether an interval between the packets P1 and P2 is a SIFS or RIFS. In other words, the receiving end cannot predict an arriving time point of the packet P2. As such, the receiving end is required to continuously monitor whether a signal that represents the preamble of a packet arises in the communication channel. More specifically, a WLAN receiving end continuously receives wireless signals in a communication channel, performs signal processes including automatic gain control (AGC), analog-to-digital conversion, demodulation and decoding, and determines whether a current input signal is the preamble of a packet according to a decoding result.
Certain signal processes performed before decoding aim at adjusting an input signal to meet requirements of subsequent processes. For example, the amplitude of an input signal is changed through an AGC process so that the amplitude of the adjusted signal conforms to an input signal range of a subsequent analog-to-digital converter (ADC). Further, certain processes are for eliminating offsets caused by channel effects or circuit mismatch, e.g., carrier frequency offset (CFO) and sampling frequency offset (SFO), so as to prevent these offsets from leading to incorrect decoding results. In practice, operation parameters, e.g., a gain of an AGC circuit, or a compensation parameter for eliminating the CFO or SFO, involved in the two types of processes above, need to be dynamically adjusted in real-time.
In the prior art, a receiving end usually stores a set of original parameters, and resets its hardware/software/firmware according to the set of original parameters each time packet searching begins (e.g., at the time point t11). For example, the set of original parameters may include an initial gain for initializing an AGC circuit. The AGC circuit then dynamically adjusts a gain applied to an input signal according to the amplitude of the input signal, starting from the initial gain. In practice, after packet searching begins and before the preamble of a next packet arises (e.g., a period between the time points t11 and t12 in FIG. 1), the input signal inputted into the receiving end is noises in the communication channel, and the adjustable parameters may drastically fluctuate and even significantly deviated from the initial values. For example, an enormous difference may exist between the AGC gain Gt12 at the time point t12 and the AGC gain Gt11 at the time point t11.
In the example in FIG. 1, after AGC control, analog-to-digital conversion, demodulation and decoding processes, at the time point t13, the receiving end determines that the preamble of the second packet P2 satisfies searching conditions. It is understood that, as the difference between the AGC gain suitable for receiving the preamble of the second packet P2 and the gain Gt12 gets larger, the AGC circuit usually takes more time for adjusting its gain, and thus the time at which the time point t13 emerges becomes delayed. Similarly, large differences between the various adjustable parameters and corresponding converged values all lead to the delay in the time point t13. With a delayed emerging time of the time point t13, a part of the data carried in the second packet P2 may be lost, or the preamble of the second packet P2 may not be identified to even miss the entire second packet P2.